Thermostatic switch



April 5 w. A. YONKERS 2,549,740

THERMOSTATIC SWITCH Filed March 5, 1948 2 Sheets-Sheet l 1 EN TOR.

April 1951 w. A. YONKERS THERMOSTATIC SWITCH 2 Sheets-Sheet 2 FiledMarch 5, 1948 INVEN mm QV hm Wm fw mm Q mm mm 3w Fm MN .3

Patented Apr. 17, 195 1 UNITED STATES TENT OFFICE THERMOSTATIC SWITCHApplication March 5, 1948, Serial No. 13,239

3 Claims.

This invention relates to temperature actuated control devices and moreparticularly to a thermostatic switch of novel construction andoperation.

Thermostatic switches as a class are well known and established in theart. In general such devices comprise a temperature sensitive elementcapable of producing a motional dis' placement related to changes in thesurrounding air temperature. The motional displacement is employed toopen or close a set of suitable contacts thereby opening or closing anelectrical power or signal circuit.

lhe adaptation of thermostatic switches to the control of relative highcurrent and/or voltage circuits presents design problems of aconflicting nature. A slow or gradual opening of this type of circuitproduces a prolonged electrical are which rapidly deteriorates thecontact material. Such destructive action may be overcome by making thecontacts sufiiciently massive to dissipate the heat generated at thepoint of the electrical arc. However, massive contacts, in turn,increase the over-all size of the device and reduce the sensitivity andoperating accuracy of the thermostat. 1

It has been proposed to solve the problem relating to heavy dutythermostatic switches by employing permanent magnets in conjunction withthe actual circuit breaking contacts, or by employing various leverageand gear mechanisms, designed to cause a sudden opening of the contactswhen the temperature reached the value for which the device is adjusted.While such devices are satisfactory for certain applications they cannotbe held to a. close operating tolerance,

that is, the temperature at which the contacts close will in time varysubstantially from the preset or adjusted value. Further, a relativelywide change in temperature is required between the contact closing andcontact opening operation.

The above problems become aggravated when the thermostatic switch is tobe designed for high temperature operation as such applications imposelimitations with respect to the materials that may be employed, theirsize, form andcoaction within the device itself.

An object of this invention is the provision of a thermostatic switchadapted for relatively high temperature use and provided with simplemeans for the snap actuation of contact elements whereby the switch maybe used to control an electrical circuit of relatively high current and/or voltage. I

An object of this invention is the provision of a thermostatic switchincluding cooperating members having different thermal expansioncoefficients and means for translating the relative motion between thesaid members into an abrupt movement of a movable contact whereby themovable contact will close and open an electrical circuit atpredetermined values of temperature.

An object of this invention is the provision of a thermostatic controlwherein a relatively small gradual motion of an expansible member istranslated into a relatively large, abrupt motion of anel-ectricalcircuit control member.

An object of this invention is the provision of a temperature actuatedcontrol comprising concentrically disposed members having difierentthermal expansion coefiicients, a stressed disc, means for imparting asudden, snap-action motion to the stressed disc at a predeterminedincrement of relative expansion between the concentrically disposedmembers, and contact members adapted. to open and close an electricalcir cuit in response to motion of the stressed disc.

An object of this invention is the provision of a temperature actuatedcontrol comprising a housing enclosing a pair of spaced terminalmembers, a movable member normally closing an electrical circuit throughthe terminal members, a conically stressed disc disposed within thehousing, means for applying a force along the circumference of the discand normal to the plane of the disc to produce an abrupt reversal in thecurvature of the disc, and means effective to displace the movablemember in accordance with the motion of the disc to thereby open theelectrical circuit through the terminal members.

An object of this invention is the provision of a high temperaturethermostatic switch adapted to open a relatively high current electricalcircuit at a preset temperature and which switch is of small size, lightweight, and of simple construction and longoperating life.

These and other objects and advantages will be apparent from thefollowing description when taken with the accompanying drawings whichare for purposes of illustration only and are not to be construedasdefining the scope or limits of the invention.

In the drawings wherein like characters refer to like parts in theseveral views: 7

Figure l is an elevation view of a thermostatic switch made inaccordance with this invention;

Figure 2 is an end view of the device;

Figure 3 is a central cross-sectional view taken along the line A-A ofFigure 2 and showing the Contact members in circuit closed position; and

Figure 4 is a cross-sectional view similar to Figure 3 and showing thecontact members in circuit open position.

Referring to Figures 1 and 2, a thermostatic switch made in accordancewith this invention comprises a base I9, casing II and a tubular shellI2 substantially as shown. The casing Ii is provided with a set ofapertured extensions I3 that serve as a means for mounting the deviceupon a piece of apparatus.

The construction and assembly details of the thermostatic switch areshown in the central cross-sectional views, Figures 3 and 4.

A subbase I4, made of suitable insulating material such as a siliconecompound which is also suitable for high temperature operation, isprovided with a pair of openings through which are inserted the contactterminals I5 made of metal. The terminals I5 are provided with integralheads I6 and'are secured in position by the mounting nuts IT, as shown.Additional nuts I8 are provided for the purpose of fastening wires froman external circuit to the terminals I5.

The inner surface of the subbase M has an axial bore within which isdisposed a cup-shaped movable contact I9 having an outer surface 20 ofplatinum, silver or other material having good electrical contactingproperties. A similar surface 2| (see Figure 4) is provided on the headsI6 of the terminals I5 as by plating, welding, etc. The compressionspring 22 provides a biasing force tending to establish a closurebetween the movable contact and the terminals. Thus, in the positionillustrated in Figure 3, a good electrical circuit is made between thetwo terminals I5.

Abutting against the subbase I4 is a metal washer 23 having a centralaperture therein, said washer 23 being spaced from the base I0 by ametallic cylinder 24. It will be apparent the subbase I4, washer 23 andcylinder 24 are firmly fixed in position when the edge 25 of the base Itis turned over the shoulder of the subbase i4, substantially as shown.

A metal ring 26 having an inwardly extending radial shoulder 2'! lieswithin the cylinder 24 and is biased against the base ID by the forceexerted by the spring washer 28 disposed between the ring 29 and thewasher 23. A stressed, snap-action disc 29 is disposed within the ring26' and rests against a circular ridge formed in the shoulder 21 of thesaid ring 26, substantially as shown. It should be noted the apex of theridge forms a circle having a diameter less than the diameter of thedisc 29, that is, the edge of the disc overhangs the ridge beyond thepoint of mutual contact. The disc 29 is preformed to have a normallyconcave curvature as shown in Figure 4, and carries an axial plunger 39that is secured thereto by a screw 3 I. The plunger 39 is made of amaterial having good electrical insulation properties and capable ofwithstanding high temperatures such as, for example, a silicone moldingcompound. A cone-shaped member 32 has its apex alined with an opening inthe base I0 and its base contacts the disc 29 circumferentially beyondthe point of contact between the disc and the ridge of the shoulder 21.

When the disc 29 has a normal concave curvature, as shown in Figure 4,the plunger 39 extends into the bore of the subbase I4 therebypreventing contact between the movable contact I9 and the terminals 2I.However, when a sufficient pressure is applied to the apex of the member32, the disc abruptly changes its curvature from concave (Figure 4) toconvex (Figure 3). The change in the curvature of the disc results in aremoval of the plunger 30 from the bore whereby the movable contact I9moves into physical contact with the heads I6 of the terminals I5 underthe action of the spring 22.

The amount of pressure required to cause snap-action of the disc 29 isprimarily a function of the disc diameter, thickness and the initialstress imparted thereto; which factors can be controlled in themanufacturing process to pro duce discs having a desired range ofoperating characteristics.

It is apparent the circular ridge on the shoul der 2'! acts as a fulcrumfor a lever whose length is the distance between the ridge and the pointof contact between the member 32 and the disc 29. When a graduallyincreasing pressure is exerted circumferentially upon the disc 29 by themember 32, the disc retains its initial concave curvature until theapplied pressure reaches a predetermined magnitude whereupon the discsuddenly snaps to a convex curvature. Upon gradually lowering theapplied pressure the disc again snaps back to its initial concavecurvature.

In a thermostatic switch adapted to control a relatively large currentit is essential that the cooperating contacts open the electricalcircuit quickly to extinguish the resultant electrical are as quickly aspossible. The work required to reverse the curvature of the disc 29 isthe product of a relatively large force multiplied by a relatively smalldisplacement representing the minute motion required to be impartedalong the edge of the disc. On the other hand, the work performed by thedisc represents a reduced force multiplied by a relatively largedisplacement of the plunger 30. Although the force applied to the discmay vary gradually, the action of the disc is sudden. Thus, the openingand closing of the electrical circuit, between the two terminals I5, inaccordance with the action of the disc 29 will be accomplished rapidlyand positively.

The operating mechanism described to this J point is enclosed in thecasing I I conveniently retained in position by turning the end 33 overthe base I9 and rolling the midsection at the point 34 to conform to theinturned end of the said base ID, as shown. The casing I I is open endedto facilitate the connection of lead Wires to the terminals I 5.

The temperature sensitive actuating element of the thermostatic switchcomprises concentrically arranged members formed of dissimilar metals,that is, metals having different thermal expansion coefiicients. Thedifference in longitudinal expansion of the members is employed to causeactuation of the prestressed disc.

One member of the actuating element comprises the tubular shell I2having a plug 35 and a cap 36 firmly secured to the outer end as bybrazing. The shell I2 is adjustably secured to the base it! by means ofthe coacting threads and the hollow nut 31 having a slightly taperedbore serves to lock the two parts in predetermined, relative position.To prevent alteration of the temperature operating range to which thedevice has been adjusted at the factory a hole is drilled through thenut 31 and the coacting threads of the shell I2 and base I9, and atapered pin 38 inserted therethrough. The adjustment of the device to adesired operating range will be described in more detail hereinbelow.

The tubular shell I2 is formed of a material having a relatively highcoefiicient of thermal expansion, such as steel, and has a uniforminside diameter. However, the outside diameter of the shell varies instages from a maximum value at the inner or threaded end to a minimumvalue at the outer or free end. This construction increases the speed ofresponse of the device as a whole as the relatively thin, outer end ofthe shell quickly reaches the temperature of the surrounding medium.

The other member of the actuating element comprises a rod 39 disposedwithin the shell 12 and spaced therefrom by the spacing bushings 40.provision of an air space between the rod 39 and the tubular shell alsoincreases the speed of response of the device as intimate contactbetween these parts would result in a transfer of heat from the shell tothe rod thereby increasing the time required for the shell to reach thetemperature of the surrounding medium. The rod 39 is made of a materialhaving a substantially zero or negligible coefficient of thermalexpansion such as, for example, Invar, a well known alloy. As shown inth drawings the rod 39 is substantially shorter than the shell l2 and asteel rod H is interposed between the rod 39 and the cone-shaped member32. As the steel rod 4i and the steel shell 52 have equal expansioncoefficients there will be no change in their relative lengthsirrespective of the temperature of the surrounding air. Thus, the upperpart of the device is insensitive to temperature changes. In otherwords, the temperature sensitive portion of the device comprises thefree end of the shell.

In assembling the thermostic switch the various components disposedwithin the base it are 8 assembled as shown in the drawings after whichthe edge of the base It! is turned inward over the shoulder of thesubbase M to provide a sub-assembly or head. The relative positions ofthe subbase l4, Washer 23 and cylinder 2 are thus fixed against furthermovement. The position of the ring 26 is also fixed except that the ringi subject to movement away from the base It) when a relatively strongpressure is exerted against the shoulder 21 to overcome the pressureexerted by the relatively heavy washer 28. However, the disc 29 and theconeshaped member 32 have freedom of axial movement within the ring 26it being again pointed out the normal curvature of the disc is concaveas shown in Figure 4.

The tubular shell l2, carrying the rods 39 and 4 l, is then threadedinto the base l0. At this stage of the assembly the lock nut 31 isthreaded well up on the thread of the shell away from the base l0.Further threading of the base l0 upon the shell [2 causes the rod 39 and4| to abut against each other and against the plug 35 and cone 32respectively. When this condition is reached a further threading of thebase on the shell causes the cone 32 to apply a gradually increasingpressure circumferentially to the disc 29. When the pressure against thedisc reaches a sufiicient magnitude the disc abruptly changes itcurvature from concave to convex (as shown in Figure 3) and any furtherincrease in pressure merely results in a flexing of the spring washer28. The sudden reversal in the curvature of the disc results in thewithdrawal of the plunger 30 from the bore in the subbase I4 therebypermitting the movable contact l9 to c ose an electrical circuit betweenthe terminals under the action of the compression spring 22.

The amount of pressure initially applied to the disc, with properconsideration given to the dimensional factors of the co-acting parts,estabill lishes the temperature at which the device will operate. Theoperating point may be checked by immersing the end of the shell E2 in acontrolled temperature bath. As the temperature of the bath is increasedthe tubular shell l2 expands gradually in axial length. Inasmuch as therod 39 has a negligible thermal expansion coefiicient its axial lengthremains fixed. Consequently, axial expansion of the shell is effectiveto reduce the pressure exerted against the disc 29 and when the pressureis reduced to a certain magnitude the disc abruptly reverses itscurvature to the normal state (concave), under which condition theplunger 38 forces the movable contact 19 out or" contact with theterminals 55.

After the thermostatic switch has been adjusted to operate at apredetermined temperature the lock nut 31 is threaded upon the extensionof the base In, a hole i drilled through the nut and the tapered pin 38forced therein.

The spring washer 28 prevents a buckling oi the coacting parts when thedevice is subjected to low temperatures during storage, etc. At lowtemperatures the shell l2 contracts in length thereby increasing thepressure applied to the disc 29 beyond the original pressure applied inthe original adjustment of the device as a whole. Such increase inpressure results in a flexing of the washer 2t and a correspondingmovement of the ring 26. Also, the washer 28 is in the form of a closedring with the center portion bowed or dished in. A closed spring washerso made has the unique property of exerting a maximum resistance todeformation when in the bowed state, and a decreasing resistance as theinner and outer sections approach a common, flat plane. Such acharacteristic is desirable in this particular application as the washereffectively decreases the mechanical loading on the mechanism atdecreasing temperature. Thus, if the parts are designed to operateproperly at the normal temperature range, a decrease in temperature toas low as -50 C. will not result in a failure of the parts.

Although a thermostatic switch made as herein described is adapted tomany apparent uses it is particularly well suited for the protection ofelectric motors having field windings insulated with a siliconecompound. The silicone insulation is capable of withstanding hightemperatures permitting operation of the motor at a higher tempera ture,in the region of 200 C., and the thermostatic control must be capable ofpositive operation at these temperatures. My thermostat is operativelymounted on the motor by drilling a hole in the motor end bell, insertingthe device through the hole so that the end of the shell 12 liesadjacent to the motor field winding. The device is secured into positionby screws or bolts passing through the apertures of the case ears l3 andinto the motor end bell. The thermostat is connected into one side ofthe line so that current flowing through the motor passes through theterminals [5 and movable contact l9. Should the motor temperature exceedthe safe limit the thermostatic switch, having been adjusted to operateat the critical value of temperature, snaps oil the current by openingthe electrical circuit between the two terminals l5. Any tendency of theelectrical contacts to stick together cannot affect the temperaturesetting of the mechanism as the plunger 30 is in rapid motion when itstrikes the movable contact 59. In the case oflarge motors thethermostatic switch contacts may control the operation of a power relayor circuit breaker designed to control the interruption of the very highmotor current.

Having now described my invention certain variations in the size, shapeand arrangement of the parts will be apparent to those skilled in theart. Such variations may be made without departing from the spirit andscope of the invention as recited in the following claims.

In the claims, the application of the pressure required to cause anabrupt change in the curvature of the stressed disc is recited as beingapplied circumferentially to the disc. It is understood that this andsimilar terminology is intended to define a force which is appliedsubstantially normal to the surface of the disc and at points forming acircle adjacent to the circumferential edge of the disc as illustratedin the structure shown in the drawings and described herein.

I claim:

1. A thermostatic switch comprising a casing open at one end and havinga central aperture in the other end, a base of insulating materialdisposed within the open end of the casing, said base having a centralbore, means securing the casing and base to prevent axial motion of onerelative to the other, a pair of terminals extending through the baseeach terminal having a contact section extending into the bore, acup-shaped contact in the bore of the base, a spring disposed betweenthe base of the bore and the cup-shaped contact to bias the said contactinto engagement with the contact sections of the terminals, a ringmember within the casing and spaced from the base said ring memberhaving a circumferential ridge, a snap-action disc in face contact withthe ridge of the ring member, a conical-ring washer disposed between thering member and the base, a cone member having its base incircumferential contact with the disc and its apex alined with theaperture in said casing, a tubular shell having a closed outer end andan open end, a rod within the she11 and extending beyond the open end,means adjustably securing the shell to the casing whereby the rod abutsagainst the apex of the cone member, and a plunger having one endsecured to the disc and the other end spaced from the cup-shaped contactwhen the curvature of the disc is convex with respect to the conemember.

2. The invention as recited in claim 1, wherein the means adjustablysecuring the shell to the casing comprises an axial extension of thecase having an internal thread co-acting with an external thread on theshell, and a hollow nut having a tapered bore, said nut threaded on tothe shell and overlapping the said extension of the case.

3. The invention as recited in claim 2 and including a transverse holethrough the nut and the co-acting threads of the shell and caseextension, and a tapered pin in said hole.

WILLIAM A. YONKERS.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,000,661 Beers Aug. 15, 19111,883,046 Spencer Oct. 18, 1932 1,919,265 Vaughn July 25, 1933 2,133,545Krieger Oct. 18, 1938 2,214,375 Jackson Sept. 10, 1940 2,230,770 VanAlmelo Feb. 4, 1941 2,303,127 Koppel Nov. 24, 1942 FOREIGN PATENTSNumber Country Date 526,786 Great Britain Sent. 25, 1940

